This invention relates to apparatus for testing rack teeth of a rack member which may be intended for use in a rack and pinion vehicle steering gear.
Conventionally, rack and pinion steering gears incorporate a rack member, the teeth of which are of constant pitch and wherein each tooth has the same uniform cross-sectional profile so that when the rack member is driven by rotation of a pinion with which it engages, a substantially constant gear ratio is provided for that drive between the pinion and the rack member. With such a rack member the teeth are easily formed by a conventional broaching operation. Since the teeth each have a uniform cross sectional profile it is a simple matter to check or test the accuracy of the finished rack teeth by use of shadow graph techniques whereby an appropriate comparison can be made between the profile of the rack teeth and a master profile of the desired tooth form from which the acceptability of the rack teeth under test can be determined.
There is increasingly a demand for vehicle steering systems having so-called variable ratio rack and pinion gears. Gears of this kind incorporate a rack member having a longitudinal axis and comprising a longitudinal array of laterally extending rack teeth, the rack teeth (which may, and usually will, extend laterally at an acute angle with respect to the said axis) being intended for driving engagement with a helically toothed pinion and comprising teeth of varying pitch having a cross sectional profile which is non-uniform over the lateral extent of those respective teeth to provide a variation in gear ratio between a said pinion and the rack member when said driving engagement is effected; such a variable ratio rack member will hereinafter be referred to as "of the type specified". Examples of rack members of the type specified are discussed in British Pat. Specifications Nos. 1,356,172 and 2,132,513, and, because of the non-uniform cross sectional profile which the rack teeth have over their lateral extent, it is not possible to machine the final form of those teeth by conventional broaching techniques. Consequently a preferred method of forming the variable ratio rack teeth is by forging or pressing between dies. Furthermore, because of their non-uniform cross sectional profile as aforementioned, it is not possible to test or check the acceptability of the variable ratio rack teeth by the conventional shadow graph techniques. It is, of course, essential that the rack teeth of rack members of the type specified are tested to ensure that they are formed to the required accuracy and within the tolerance of acceptability for a steering gear of which they may be intended to form part and there is a requirement for an apparatus by which the acceptability of the rack teeth can readily and efficiently be determined it is an object of the present invention to satisfy this requirement.